Battery separators serve as electronic insulators and ionic conductors, i.e., they prevent the direct electronic contact of electrodes of opposite polarity while enabling the flow of ionic current between them. To meet these two functions, separators are usually porous insulators with pores as small as possible to prevent electronic short circuits by dendrites or plate particles and with a porosity as high as possible to minimize the internal battery resistance. In lead-acid batteries, the separator may also determine the proper plate spacing and thereby may define the amount of electrolyte which participates in the cell reaction. The typical separator has to be stable over the life time of the battery to withstand the highly aggressive electrolyte and oxidative environment.
Beyond these basically passive functions, separators in lead-acid batteries can also actively affect the battery performance in many ways. There are two general classes of lead-acid batteries: flooded cell batteries and valve-regulated lead-acid, commonly abbreviated as VRLA, batteries. In flooded cell batteries, the battery separators typically include porous derivatives of cellulose, polyvinyl chloride (PVC), organic rubber, or polyolefins. Microporous polyolefin separators, such as those marketed by Daramic, LLC of Charlotte, N.C., are often preferred and may contain silica filler which may improve wettability.
For VRLA batteries, two technologies are predominant: batteries with an absorbent glass mat (AGM) and gel separators. The absorbent glass mat, which may be either woven or non-woven, both holds the electrolyte and functions as a separator. In gel batteries, the electrolyte is gelled and immobilized by, for example, fumed silica. Compared to AGM batteries, the manufacturing cost of gel batteries is higher and the specific power is lower due to a higher internal resistance.
In VRLA AGM batteries, electrolyte is absorbed into the AGM separator and held there during the operation of the battery. Material for such AGM battery separators is commonly a glass mat made of insoluble microtine glass fiber materials. Typical specific gravity of the dilute sulfuric acid in a sealed lead acid (SLA)-VRLA battery ranges from about 1.265 to 1.36, and may typically be about 1.28 for VRLA.
The cells of the VRLA AGM battery are each made up of AGM separator, positive and negative electrodes (or plates), and electrolyte. The AGM separators may be wrapped around the positive plate at the bottom to prevent bottom shorts between the positive and negative plates and may extend beyond the side edges of the plate to prevent side shorts and also beyond the top edge if desired. The typical AGM separator is made of almost entirely microfibers of glass (may include coarse and fine fibers) or may contain microfibers of glass and some synthetic fibers. Typical AGM separators are insoluble in water, sulfuric acid or electrolyte. The space formed between glass fibers commonly known as pores or porosity absorbs and retains electrolyte (dilute sulfuric acid or battery grade sulfuric acid) and does not have any extra acid (electrolyte) outside the separator and therefore there is no acid spill in the AGM batteries. This acid starvation allows AGM batteries to be held in either horizontal or vertical positions with no fear of acid spilling or damaging the straps or contacts on the top.
The assembly containing positive and negative electrode with AGM separator in between is typically compressed at about 10 kPa pressure and loaded into the battery enclosure or case. The final AGM separator thickness after compression is lower than the initial thickness and therefore, the final porosity after compression is lower than initial porosity. This compression of the AGM separator is required to keep the positive active material on the positive plate from shedding during operation of the VRLA AGM battery, to keep the AGM separator intact as it is made of loosely held microfine glass fibers, and to maintain the integrity of the battery.
After pressing the AGM separator between electrodes (or plates) and placing the electrode and separator cells inside the battery container, the battery is filled with a predetermined volume of acid (of electrolyte). However, this compression of the AGM separator works against the acid filling process. The reduced thickness and lower porosity of the compressed separator slow down the acid filing process. Such a slow filling process not only increases the cost of battery production, but also can harm the plates and reduce battery function. The pores are the regions that allow acid to be absorbed and held. Although AGM separators have a high porosity (>90% in general, pre-compression), the porosity reduction due to compression works against the speed at which the battery can be filled with acid. If the filling, wetting or soaking ability of the AGM separator is low (due to high compression induced porosity reduction), the top section of AGM separators (when acid is added from the top of the battery container) soak first holding acid which in turn comes in contact with negative plate initiating sulfation (which otherwise happens during discharge of the battery). The premature sulfation of the negative plate results in capacity reduction of the battery (regions not charged when the battery is formed). Therefore the lower porosity and reduced thickness of the compressed AGM separator results in a slow acid filling process with consequent low productivity battery fabrication process, less acid between the plates, and the differential acid contact with the electrodes (top sections over the bottom sections of the electrodes) with consequent sulfation and reduction in battery capacity. The reduction in battery capacity may be the most critical issue with slow acid filling.
Also, if the AGM separator is not sufficiently porous, the electrolyte will not move efficiently through the separator. As a result, filling the area between the electrodes can be difficult. If the separator is not completely saturated with electrolyte, portions of the electrodes may not be in contact with the electrolyte and may not provide ionic conduction. Reduced contact between the electrolyte and electrode may reduce the capacity delivered by the battery and eventually cause the battery to fail. Increased problems including sulfation arise due to longer acid fill times or partial electrolyte fill.
The electrolyte has to displace the air in the pores before occupying the pores. This slow displacement process results in slow acid filling process of the compressed AGM separator. This may be made quicker by being assisted by vacuum. Current VRLA AGM battery manufacturing processes typically require about 30 minutes to fully absorb, fill or wick the electrolyte (acid) into the compressed glass mat, and usually the application of a vacuum is used to slightly accelerate the fill process. Hence, even with vacuum assist, there is still a need for improved absorbent glass mats, improved VRLA batteries, better acid fill rates, and the like. And it is still desirable to minimize the acid fill time (to increase the fill rate) to produce lower cost, higher capacity or longer cycle life VRLA AGM batteries.